An alternative solution has been to provide a shrouded carbon/graphite seal face ring, with vent holes, to equalise pressure across the seal ring, for example as disclosed in GB2,296,052. This solution however has the disadvantage that it increases the size of the seal significantly.
A further alternative solution has been to shrink fit a metal shroud to the outside diameter of the seal face ring. Because of the high differential thermal expansion between the components, the amount of pre-compression induced on the seal face reduces significantly as the temperature increases, thus limiting the internal pressure capability. Furthermore, the pre-compression induces out of roundness of the seal face ring, causing a wavy distortion to the mating surface. This waviness becomes apparent as the temperature of the ring is changed from that at which the seal face has been lapped.